Question: 10 rubber stamps cost $16.40. Which equation would help determine the cost of 7 rubber stamps?
Explanation: There are several equations that could help determine the cost, each with a slightly different approach. Let $x$ represent the unknown cost of 7 rubber stamps. Since 7 rubber stamps cost $x$ , we have the following proportion: $\dfrac{7}{x}$ We can write the fact that 10 rubber stamps cost $16.40 as a proportion: $\dfrac{10}{\$16.40}$ The cost changes along with the number of rubber stamps purchased, and so the two proportions are equivalent. $\dfrac{7}{x} = \dfrac{10}{\$16.40}$